Electrophoretic coating of metal substrates using elevated pressures

ABSTRACT

In a process of electrophoretic coating of electrically conductive surfaces with organic resins, the amount of resin deposited is substantially increased by conducting the electrophoretic deposition at pressures above atmospheric.

United States Patent Frost et al. 1 June 6, 1972 [s41 ELECTROPHORETIC COATING 0F [56] References Cited METAL SUBSTRATES USING UNITED STATES PATENTS ELEVATED PRESSURES 3,476,666 11/1969 Bell et al ..204/181 1 lnvemors= Lincoln La Salle; Amd Sladler- 3,496,082 2/1970 Orem et al. ..204/l81 Park Forest, both of ill.

Primary Examiner-Howard S. Williams [73] Asslgnee' Continental can Company New Attorney-Paul Shapiro, Joseph E. Kerwin and William A.

York, N.Y.

Dittmann [22] Filed: July 1, 1970 [2]] Appl. No.: 51,692 [57] ABSTRACT In a process of electrophoretic coating of electrically conductive surfaces with organic resins, the amount of resin [52] "204/181 deposited is substantially increased by conducting the elec- [51 Int. Cl. "801k 5/02, C23!) 13/00 trophoretic deposition at pressures above atmospherie [58] Field of Search 4/181 5 Claims, No Drawings ELECTROPHORETIC COATING OF METAL SUBSTRATES USING ELEVATED PRESSURE'S BACKGROUND OF THE INVENTION 1. Field of the invention This invention relates to coating the electrically conductive surface areas of articles, and in particular, to coating metal articles by electrophoretic deposition of organic film-forming material.

2. The Prior Art In the manufacture of cans,- the blank stock is ordinarily coated while the material is flat. Since the cans are welded afier this operation, the edges of the blank stock are cleaned so that the welding process forms an impervious joint. After welding, it is necessary to apply side striping or coating to the exposed cut edges of the metal portions that form the can sideseam to prevent corrosion and/or product contamination on the interior portion of the can and for esthetic appeal on the exterior portion of the can body.

One method that has been proposed for coating these exposed bare metal surface areas of the can side-seam is electrophoretic deposition. in this coating method, the metal surface to be coated is immersed in an electrophoretic bath which generally consists of particles of an organic-film forming polymeric coating materials which are suspended in an aqueous electrolytic solution. The exposed bare conducting surface areas of the surface to be coated serve as one electrode of the electrocoating bath. When a potential of the required polarity is applied between the exposed surface areas of the article to be coated and a second electrode which completes the electrical circuit and is dipped into the aqueous coating suspension, the particles of the coating material are deposited on the metal surfaces exposed areas in the form of a uniform layer.

Electrophoretic deposition has not been practiced to date for coating the bare cut edges of can side-seams due to the fact that the coating thicknesses required i.e. about6 to 8 mils cannot be achieved using conventional electrophoretic deposition processes. Using such conventional electrophoretic coating techniques the maximum coating thickness that can be achieved are in the range of 2 to 4 mils.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a method for the electrophoretic coating of metal surfaces which includes contacting the surface to be coated with an electrophoretic bath having a synthetic polymer resin dispersed therein and electrophoretically depositing the synthetic polymer resin on the exposed electrically conductive areas of the surface at a pressure greater than atmospheric.

As will hereinafter be illustrated, the step of electrophoretically depositing the synthetic polymer resin at pressures greater than atmospheric results in a deposited coating of substantially greater thickness when compared to similar coated surfaces which have not been subjected to elevated pressures.

PREFERRED EMBODIMENTS To effect the electrophoretic coating of metal surfaces in accordance with the present invention, a synthetic polymer resin containing electrolyte is contacted with the metal surface to be coated. To efi'ect deposition of the polymeric coating material from the electrophoretic coating bath, an electrical current is caused to flow between the metal surface to be coated and a suitable electrode immersed in the electrophoretic coating bath by establishing a DC potential between the metal surface and the electrode and permitting the electrical current to flow until the electrically conducting surface areas of the metal have been coated with polymer deposited from the bath. in the case of can bodies which have been enamel coated prior to exposure to the-bath, the electrophoretic polymer coating is deposited only on those areas of the enamel coated bodies where bare metal is exposed such as the cut edges of the metal portions that form the side seam of the can. Due to the electrical insulating properties of the enamel coating, no polymer is electrodeposited on the enamel coated areas of the can body.

During the electrodeposition process in order to achieve coating deposits of substantially increased thickness in accordance with the process of the present invention, the pressure of the atmosphere surrounding the article surface immersed in the electrophoretic bath is raised above atmospheric pressure. Generally the pressure required to achieve the improved results is maintained in the range of about 1 to about 50 pounds per square inch (psi), pressures in the range of 2 to 20 psi being preferred.

The electrical potential applied in the electrophoretic bath to deposit the repair coating is controlled below the value at which dielectric breakdown of the deposited coating occurs and may range from between about 20 volts to about 2,000 volts per inch of electrode spacing.

The time required to effect repair coating is not critical since coating of the exposed portions of the metal surface to be coated begins in a fraction of a second from the point where the electrical circuit is closed and stops when the insulating polymeric coating is deposited. Thus, the time used to effect the deposition of the coating may range between about 1 second and about 100 seconds and preferably about 2 to 20 seconds.

The current density selected for the electrophoretic deposition varies and generally will depend upon such parameters as the selected voltage, the conductivity of the electrophoretic coating bath, and the time allotted for the formation of a coating. Generally, the electrophoretic processing voltage is varied from about 100 to about 900 volts per inch of electrode spacing to coat exposed metal areas within a reasonable time, e.g., about 2 to about 5 seconds.

The temperature of the electrophoretic bath is not critical other than the boiling and freezing point of the colloidal suspension used as the electrophoretic bath. For practical applications it is preferred to use temperatures in the range of about 15 to about 60 C. i

The electrophoretic coating bath used in the process of the present invention is a collodial suspension consisting of charged particles of a synthetic polymer resin dispersed in a suitable liquid electrolyte. Preferably, the resins utilized in the electrophoretic coating baths of the present invention are any resins, thermoplastic or thermosetting, which have free carboxylic acid groups or hydroxyl groups.

Illustrative examples of resins which have free carboxylic acid or hydroxyl groups which may be used in the electrophoretic baths include acrylic resins such as polyacrylic acid, polymers of hydroxyalkylesters of 01,3 -ethylenically unsaturated carboxylic acids, such as poly (Z-hydroxyethyl acrylate), poly (2-hydroxypropyl methacrylate), copolymers of of ethylene and 11,13 -ethylenically unsaturated carboxylic acids, such as ethylene/acrylic acid copolymers, ethylene/methacrylic acid copolymers, polymers of monoand di-esters of unsaturated dicarboxylic acids, such as maleic acid, fumaric acid, and itaconic acid, in which at least one of the esterifying groups contains a hydroxyl group, such as polymers of mono (2-hydroxyethyl) maleate, mono (2-hydroxypropyl) furmarate, and the like.

The above described resins may be further blended with an alkoxy methyl aminoplast, such as a methoxy melamine ester, methoxy methyl benzoguanamine ether, or butylated urea/ formaldehyde resin to obtain heat hardenable resin formulations containing about 60 to percent 'of the carboxylic acid or hydroxyl group containing resin and about 5 to about 40 percent by weight of the aminoplast resin.

A stable dispersion of these resins in water is made in the presence of ammonia or triethanolamine. An anionic surface active agent may be utilized if necessary.

The concentration of resin solids in the electrophoretic bath may be widely varied. In general, the solids content of the synthetic polymer resin in the bath may be varied from 2 to 15 percent by weight.

After the synthetic polymer resin is electrophoretically deposited on the metal surface, the article is separated from the bath and rinsed with water to remove adhering portions of the coating solution. Following rinsing, the electrophoretically coated article is placed in an oven and baked at an elevated temperature, e.g., about 300 to about 450 F for about 1 to about 20 minutes to remove all volatile material and cure the electrophoretically deposited coating to a hardened film.

To illustrate the manner in which the present invention may be carried out, the following examples are given. It is to be understood, however, that the examples are for the purpose of illustration, and the invention is not to be regarded as limited to any of the specific materials or conditions recited therein.

EXAMPLE I Blanks were cut from an epoxy-urea/formaldehyde resin coated metal sheet and fabricated into open cylinders having a welded side seam to form the body of a container wherein the enamel coated side of the blanks formed the interior of the container. The cut edge of the metal portions that formed the seam was bare metal. The container body was closed at one end with end closures formed from a similarly enamel coated sheet which was double seamed to the cylinder wall.

The container was then filled with electrophoretic bath consisting of percent resin solids dispersed in an aqueous triethanolamine solution. The resin solids were composed of a mixture of 80 percent by weight of a 70% hydroxyl functional acrylic resin having a molecular weight of about 30,000 and 20 percent by weight of a melamine/formaldehyde resin.

A steel electrode was placed in the container in a position so that all the points on the electrode surface were V4 inch to l 9% inches from the nearest point on the inside of the container. The container was then placed in a pressure vessel and the vessel closed.

A current having a potential of about 465 volts was applied across the electrode in the container for about 4 seconds, the container being the anode. The temperature of the bath was about 1 F after the application of the current, the container was inverted to removethe bath liquid. The inverted container was rinsed with water and baked for 2 minutes at 250 F and 4 minutes at 385 F in a forced air oven.

A series of similarly fabricated containers .were coated following the above procedure using pressures varying from 2 to 'psi.

For purposes of comparison, the above coating procedure was also conducted at atmospheric pressure (0 psi). The coat ing weights deposited are recorded in table 1 below. A coating weight of about 70 milligrams is approximately equivalant to a deposited coating thickness of 8 mils.

TABLE 1 Test No. l 2 3 4 Control Pressure (psi) 2 5 i0 20 O Coating Weight Gain (mgs.) 45 48 57 7O 37 By reference to table I it is immediately apparent the cans electrophoretically coated at elevated pressures in accordance with the present invention show a substantially greater gain in coating weight when compared to similar cans coated under ambient or atmospheric pressures.

The procedure of example I was repeated with the exception that the acrylic resin used 'in the electrophoretic bath had a molecular weight of about l5,000. In this test series the effect of subatmosphenc pressure on deposition rate was also studied. The results of this series of tests is recorded in table ll below. Control tests (0 psi) and subatmospheric pressure tests are designated by the symbol C.

The procedure of example I was repeated with the exception that Shell DX-3 l the reaction product of an epoxy resin, bisphenol A, benzoic acid and a linseed oil fatty acid was used as the resin vehicle in the electrophoretic bath at a solids content of 9.8 percent. The results of this series of tests is recorded in table Ill below.

TABLE III Test 8 9 10 1 1 C, No. Pressure 5 10 20 50 0 (p Coating 62 66 76 98 54 Weight Gain mgs.

What is claimed is:

l. A method of electrophoretically coating articles having electrically conductive surfaces which comprises the steps of:

a. causing the electrically conductive surface of the article to be coated to be immersed in an electrophoretic bath having charged particles of an organic coating material dispersed therein, the electrically conductive article surface serving as a first electrode;

b. raising the pressure of the atmosphere above the bath to a pressure greater than atmospheric pressure and then;

c. causing a second electrode to contact said electrophoretic bath and causing a direct electric current to flow in the bath between the electrically conductive article surface and the second electrode until the electrically conducting surface areas of the article have been electrophoretically coated with the organic coating material.

2. The method of claim 1 wherein the pressure in the atmosphere above the bath is raised between about 1 to about 100 psi during the electrophoretic coating of the article.

3. The method of claim 1 wherein the organic coating material is a thermoplastic resin.

4. The method of claim 1 wherein the organic coating material is a thermosetting resin.

5. The method of claim 4 wherein the thermosetting resin electrophoretically coated surface is heated to harden the electrophoretically deposited coating. 

2. The method of claim 1 wherein the pressure in the atmosphere above the bath is raised between about 1 to about 100 psi during the electrophoretic coating of the article.
 3. The method of claim 1 wherein the organic coating material is a thermoplastic resin.
 4. The method of claim 1 wherein the organic coating material is a thermosetting resin.
 5. The method of claim 4 wherein the thermosetting resin electrophoretically coated surface is heated to harden the electrophoretically deposited coating. 